What actually happens in a DDOS attack?

Is there no ordering for incoming requests that would just slow a server down instead of breaking? What actually breaks?

Imagine a server is a store with staff that handle customer requests in a first in / first out kind of order. Now imagine you’ve got 20 years worth of black Friday traffic trying to get to it.

The server might be able to handle the requests that get to it, but the sheer volume of traffic makes it impossible to get in or out effectively.

20 years of Black Friday traffic because some anonymous person put up realistic looking advertisements all over the city offering 90% off from 9:00 to noon.